Devices such as power shovels, loaders, bulldozers, hydraulic lifts, and the like rely on hydraulic cylinders and motors in order to perform their various functions. The hydraulic cylinders or motors are powered by a hydraulic pump, such as a swash plate pump, which is connected through a fluid control valve generally operated directly or indirectly by manually manipulated handles, cables or the like which control flow of hydraulic fluid to the hydraulic cylinders or motors.
The directional control valves generally include a body having a pressure port which is connected to the pump; tank ports which are connected to a tank or reservoir for hydraulic fluid, and work ports connected to one or more hydraulic cylinders. The operating devices selectively connect various ports with one another in order to control operation of hydraulic cylinders so that fluid is delivered to the cylinders and exhausted from the cylinders in accordance with the operator's purposes. Fluid control valves under consideration with respect to this invention include a body having a bore formed therein which receives a spool with a plurality of circumferential grooves thereon. The various ports are in communication with the bore via passageways which are selectively connected by positioning the spool axially with the bore.
Generally, directional control valves are classified as open center systems, closed center systems, and load sensing systems. Open center systems are relatively inexpensive, uncomplicated, and imprecise, whereas closed center systems are responsive and precisely controllable but relatively expensive. Both open and closed center systems tend to be inefficient. Load sensing systems, which are the subject of this invention, tend to be relatively efficient because the pump which generates the flow of fluid to the fluid control valve delivers that fluid at a variable flow rate and at a variable output pressure based upon the instantaneous requirements of the device controlled by hydraulic cylinders connected to the directional control valve. This is accomplished by providing a feedback signal to the pump which is representative of the fluid pressure required to operate the control device and controlling the output pressure from the pump to assume a predetermined magnitude greater than the feedback signal. In that the predetermined pressure differential between the operating pressure and required pressure is relatively small, the efficiency of a load sensing hydraulic system is much higher than the efficiency of open center and closed center systems. Directional control valves having a compensating structure for controlling the pressure differential thereacross, and consequently the flow of fluid thereto, are generally referred to as load sensing or pressure compensating valves.
The load sensing or pressure compensating valve may be either a pre-pressure compensated valve or a post-pressure compensation valve. In post-pressure compensated valves, the compensator is positioned between the spool and the output work port of the fluid control valve to regulate the pressure of the fluid supplied from the spool to a predetermined magnitude less than the pressure of the fluid at the inlet pressure port but greater than the pressure of the fluid in the active work port. Accordingly, a constant pressure differential is maintained across the spool, resulting in a constant flow of fluid therethrough, regardless of changing load requirements. A number of post-pressure compensator structures are known in the art; however, these known arrangements are rather complicated and/or require a number of components, and therefore are relatively expensive or difficult to service. Moreover, employment of post-pressure compensators can be further improved by having the components function so that maximum system operating pressure is adjusted, whereby maximum pump output flow is achieved at maximum system operating pressure.